The use of zinc oxide as a sunscreen is well known in the art. It has excellent UV screening capability which covers the full range of UVA and UVB wavelengths. It is also considered one of the safest and most trusted UV actives. Additionally, it is highly light stable and shows virtually no degradation on exposure to light.
For example, U.S. publication number 20030161795 and U.S. Pat. No. 5,032,390 describe zinc oxide particles for use in shielding the skin from ultraviolet radiation relying on physical UV screening agents alone such as zinc oxide.
Furthermore, it is well known to coat zinc oxide in order to provide it with a hydrophobic coating improving the feel on skin and helping to disperse the particles within various formulations.
In particular U.S. Pat. Nos. 6,045,650, 5,486,631, 5,756,788 describe siloxane treated metal oxides resulting in a hydrophobic metal oxide which is non-reactive, unaffected by water and can be applied to the skin for protection from ultraviolet light of the sun. PCT application 2006/1 05600 also teaches the coating of metal oxide particles via treatment of the metal oxide particles with an aqueous solution of water-soluble organosilicon monomers and initiating polymerization.
The above cited patents and application are concerned with nano sized particles, that is particles which are characterized by a size of less than 100 nm. Particles of zinc oxide in particle size ranges above 100 nm are known to cause a whitening effect on the skin. The very small particle size zinc oxide (<100 nm) allows for the incorporation of significant amounts of zinc oxide while maintaining formulation transparency on skin, a highly desirable effect.
However, in recent years the public and regulatory agencies have expressed concern regarding the use of nanoparticles (particles <100 nm) in personal care formulations. Questions relating to their absorbance into skin, their ability to impact at the cellular level and their intrinsic toxicity versus that of their macro counterparts are being considered. No definitive answers are presently available.
Additionally, the formulation of nano zinc oxide particle (<100 nm) provides numerous challenges. The surface area is extremely large and even with adequate coating of the oxide with hydrophobic materials, the particles tend to agglomerate requiring additional cost and processing steps to fully disperse to ensure that the particles apply evenly as sun screen. It is important to provide particles which are more easily dispersed.
Accordingly, there is still a need for zinc oxide particles which are of an acceptable whitening effect in formulation but of a particle size distribution which excludes particles smaller than 100 nm.
Furthermore, numerous studies in recent years have shown that ultraviolet radiation in the wavelength range from 320 to 400 nm (UV-A range) makes a significant contribution to the skin damage caused by sunlight. Therefore there is an increasing requirement for adequate protection against UV-A radiation. In addition, the availability of sun protection preparations having a high sun protection factor (hereinafter also referred to as SPF) has led to concerns that users are able to stay in the sun for longer and, as a result, are exposed to an increased amount of UV-A radiation.
Accordingly it would be highly desirable to achieve higher UV-A absorbance in combination with zinc oxide particles of a size distribution which excludes particles smaller than 100 nm.